Preheat oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Cook the jumbo shells according to package directions until al dente (usually 8–10 minutes). Drain and lay shells on a lightly oiled sheet to prevent sticking.

Finely chop the onion.

Heat a skillet over medium-high heat. Add a drizzle of oil and sauté the chopped onion until translucent, about 3–4 minutes.

Add ground beef. Break it up and brown thoroughly. Cook until no pink remains and juices run clear. Drain excess fat if desired.

Stir in salsa and simmer 1–2 minutes to blend flavors. Remove from heat.

Mix in the shredded cheddar and Monterey Jack while the meat is warm so the cheeses melt into the filling. Taste and season with salt and pepper as needed.

Spoon meat-cheese mixture into each jumbo shell, about 2–3 tablespoons per shell, depending on shell size. Arrange filled shells seam-side up in a lightly greased baking dish.

Sprinkle additional cheese over the shells to your liking.

Bake in the preheated oven for 12–15 minutes, until cheese is melted and edges are bubbling. If you like a golden top, broil for 1–2 minutes more while watching closely.

Let rest for 5 minutes before serving so the filling sets.

Garnish with fresh cilantro, sliced green onions, diced tomatoes, or a dollop of sour cream. Serve with lime wedges.